This invention relates generally to interferometers and more particularly to a refractively scanned interferometer capable of utilization as a Fourier transform interferometer/spectroradiometer.
One of the most common arrangements for performing the interferometric function is known as the Michelson Configuration of which there are many derivatives. These optical interferometers use light interference in spectroscopy and have two beams of light in separate optical paths that are directed toward a common point at which the beams meet and form interference fringes.
To achieve the ends of increasing the scan speed and resolving power for low level light intensities while maintaining excellent wave length accuracy with a minimum of stray radiant energy problems, the use of Fourier transform spectroscopy is indicated with Michelson type apparatus. Devices with Fourier transform capability, when used for radiometry, i.e., absolute intensity measurements, generally involve the detection and measurement of radiant electro-magnetic energy in the optical spectrum; therefore, the response of detectors, appropriate for the particular region of the spectrum to be investigated, can be speedily converted into a conventional spectrum with a Fourier transform computer system.
Accomplishment of the scanning for Fourier transform interferometer/spectroradiometers with the Michelson type apparatus most often included reflective scanning. The equipment utilizing reflective scanning is usually large, heavy, expensive laboratory apparatus which is not suitable for many field uses. The critical dimensional relationship between the elements of the apparatus using reflective scanning is the factor predominantly responsible for the limitations enumerated. For example, a wobble of the linear reciprocation of the reflective scanning member causes wave front misalignment which, when it exceeds about one arc-second, causes the disappearance of useful fringes.
Refractive scanning has been suggested to overcome the aforementioned disadvantages of reflective scanning. One mode involved a linearly scanned, wedge-shaped window to create a differential in optical path length, the varying thickness of the wedge-shape resulting in a proportional optical retardation. The linear reciprocating wedge scheme introduces design and manufacturing complications not present with rotary systems. Additionally, the linear reciprocation concept has been found to be sensitive to external forces from a hostile environment.
The taper of the wedge results in an increase in the cost of manufacture over that of a flat plate. It should be noted that the reciprocating action of the wedge allows for only two spectra per reciprocation, which limits the speed of data acquisition.
Another concept, discarded because of its disadvantages, involves the use of a rotatable refractive plate for scanning in one arm or optical path of a Michelson type apparatus. The disadvantages are the added expense of a compensator window for optical balancing of the other arm or the optical path which does not have the scanning plate, and the large non-linearities introduced by the optical retardation vs. rotation angle which require expensive, complex electronics and a complex control of the drive in order to compensate.
Neither the prior art devices nor the contemplated solutions has allowed for the production of a small, portable, inexpensive device which is accurate even though it is subjected to a hostile environment. Minimization of the non-linearities has thus far been an elusive goal unattainable by designs produced or suggested.